Such speakers are generally known. FIG. 8 of the accompanying drawing shows a known loudspeaker. This speaker has a frame (1), a membrane (2) and an electromagnetic drive unit (4). The membrane is formed by a conical body (2a) and has a flat outer circumferential edge (2b) and a flat inner circumferential edge (2c). The drive unit is provided with a stationary part (4a) and a movable part (4b). The stationary part, which includes a permanent magnet and a magnetic yoke, is secured to the frame. The movable part includes a voice coil and a cylindrical coil support (4b1). At its outer circumferential edge, the membrane is connected to the frame by means of a flexible suspension (6) and at its inner circumferential edge, it is adhered to the coil support, which in its turn is connected to the frame by means of a spider (8).
The conventional conical body of the membrane of the depicted known loudspeaker has a certain height in order to obtain sufficient stiffness. The membrane should have a certain minimal stiffness in order to be able to move like a piston for low-frequency reproduction and to have a controlled behavior at and above the first break-up of the membrane for mid and high-frequency reproduction. For this reason, there arise problems relating to the speaker's performance if a shallow speaker, i.e. a speaker having a small height, is required in certain applications.
JP-A 54-6523 discloses a shallow speaker, which is provided with a faintly conical diaphragm having a corrugation structure for improving the behavior of the diaphragm. The diaphragm has an outer circumferential part glued to an elastic suspension ring and an inner circumferential part glued to a voice-coil bobbin. The corrugation structure is formed by radial corrugations which decrease towards the outer circumferential part and increase towards the inner circumferential part. Due to this structure, the outer circumferential part includes a flat ring-shaped outer edge glued to the suspension ring, and the inner circumferential part includes an undulating inner edge glued to the bobbin. The speaker is further provided with a spider secured to the voice-coil bobbin.
Although the speaker known from JP-A 54-6523 comprises means for solving the problem relating to lack of stiffness of a faintly conical diaphragm, it causes another problem, viz. a problem relating to the instability of the diaphragm movements due to the small axial distance between the suspension means formed by the suspension ring and the spider.